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PAX P olarized A ntiproton E x periment

PAX P olarized A ntiproton E x periment. Status report. PAX Collaboration www.fz-juelich.de/ikp/pax Spokespersons: Paolo Lenisa lenisa@mail.desy.de Frank Rathmann f.rathmann@fz-juelich.de. Outline. Extracted beam vs internal target (vs collider) Transversity measurement by Drell-Yan

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PAX P olarized A ntiproton E x periment

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  1. PAXPolarizedAntiproton Experiment Status report PAX Collaboration www.fz-juelich.de/ikp/pax Spokespersons: Paolo Lenisa lenisa@mail.desy.de Frank Rathmann f.rathmann@fz-juelich.de P. Lenisa - Univ. Ferrara and INFN

  2. Outline • Extracted beam vs internal target (vs collider) • Transversity measurement by Drell-Yan • Rates • Angular distribution • Background • Detector concept • Conclusions

  3. Extracted beam vs internal target Lext=txNpbar t = areal density (15 g/cm2 NH3) t = areal density f = revolution frequency Npbar = number of pbar stored in HESR Lint= t xf xNpbar Drell-Yan events rate: NDY=L x sDY Polarized beam luminosity: Extracted beam: Production rate of polarized antiprotons (tP = 2 tB) cannot exceed: Npbar = 1.0×107/e2 = 1.3×106 pbar/s Lext=7.5×1024x1.3×106 = 1.0 ×1031 cm-2 s-1 Internal target Lint= 7.2×1014x 6×105x 4.9×1010 = 2.1 ×1031 cm-2 s-1

  4. Extracted beam vs internal target Statistical uncertainty in ATT d = diluition factor Q = proton target polarization P = antiproton beam polarization Extracted beam: d=3/17Q=0.85 P=0.3 Internal target: d=1 Q=0.85P=0.3 factor 67 in measuring time!

  5. l+ q2=M2 l- q qT p p qL 1) Events rate. 2) Angular distribution. Transversity measurement with Drell-Yan lepton pairs Polarizedantiproton beam → polarizedproton target (both transverse)

  6. 2 k events/day 22 GeV 22 GeV 15 GeV 15 GeV M>4 GeV M>2 GeV M (GeV/c2) • x1x2 = M2/s Drell-Yan cross section and event rate • M2 = s x1x2 • xF=2qL/√s = x1-x2 • Mandatory use of the invariant mass region below the J/y (2 to 3 GeV). • 22 GeV preferable to 15 GeV

  7. Collider ring (15 GeV) L > 1030cm-2s-1 to get the same rates

  8. The asymmetry is maximal for angles =90° • The asymmetry has a cos(2f) azimuthal asymmetry. ATT asymmetry: angular distribution The asymmetry is large in the large acceptance detector (LAD)

  9. Theoretical prediction Asymmetry amplitude Angular modulation 0.3 0.25 LAD T=15 GeV 0.2 T=22 GeV 0.15 FWD: qlab < 8° LAD: 8° < qlab < 50° P=Q=1 Anselmino, Barone, Drago, Nikolaev (hep-ph/0403114 v1) 0 0.2 0.4 0.6 xF=x1-x2

  10. Estimated signal LAD LAD • 120 k events sample • 60 days at L=2.1× 1031 cm2 s-2, P = 0.3, Q = 0.85 • Events under J/y can double the statistics. • Good momentum resolution requested

  11.  108-109 rejection factor against background Background • DY pairs can have non-zero transverse momentum (<pT> = 0.5 GeV) • coplanarity cut between DY and beam not applicable • Background higher in the forward direction (where the asymmetry is lower). • Background higher for m than for e (meson decay) •  hadronic absorber needed for m  inhibits additonal physics chan. • Sensitivity to charge helps to subtract background from wrong-charge pairs •  Magnetic field envisaged

  12. Background for … Average multiplicity: 4 charged + 2 neutral particle per event. Combinatorial background from meson decay. Prelim. estimation of most of the processes shows background under control.

  13. Background for Total background Background origin x100 x100 x1000 x1000 e e m m Preliminary PYTHIA result (2×109 events) • Background higher for m than for e • Background from charge coniugated mesons negligible for e.

  14. Detector concept • Drell-Yan process requires a large acceptance detector • Good hadron rejection needed • 102 at trigger level, 104 after data analysis for single track. • Magnetic field envisaged • Increased invariant mass resolution with respect to simple calorimeter • Improved PID through E/p ratio • Separation of wrong charge combinatorial background • Toroid? • Zero field on axis compatible with polarized target.

  15. 800 x 600 mm coils • 3 x 50 mm section (1450 A/mm2) • average integrated field: 0.6 Tm • free acceptance > 80 % Sperconducting coils for the target do not affect azimuthal acceptance. Possible solution: 6 superconducting coils (8 coils solution also under study)

  16. Conclusions • Internal target ideal to fully exploit the limited production of polarized antprotons • 22 GeV preferred to 15 GeV • Angular distribution of events mainly interests large acceptance detector • Electrons favoured over muons for additional physics • Background seems not a problem, but more detailed studies necessary • A toroid magnet might be the proper choice for the polarized target. • The collider represents an attractive perspetive (background to be studied). P. Lenisa - Univ. Ferrara and INFN

  17. Background for Example M > 2GeV 0.01 wrong charge 10 nb @ GeV2 Dalitz veto through unpaired e Combinatorial background from meson decay: Direct estimation of candidate processes shows negligible contribution.

  18. Longitudinal Field (B=335 mT) H PT = 0.845 ± 0.028 D HERMES Transverse Field (B=297 mT) H PT = 0.795  0.033 HERMES Performance of Polarized Internal Targets HERMES: Stored Positrons PINTEX: Stored Protons H Fast reorientation in a weak field (x,y,z) Targets work very reliably (many months without service)

  19. Detector Concept • Two complementary parts: • Forward Detector • ±80 acceptance • unambiguous identification of leading particles • precise measurement of their momenta • Large Acceptance Detector • measurement of angles (θ,φ) and energies of Drell-Yan pairs

  20. T = 15 GeV T = 22 GeV Count rate estimate Uncertainty of ATT depends on target and beam polarization (|P|=0.05, |Q|~0.9) For single spin asymmetries L ~ 10 times larger resonant J/Ψcontribution (2  higher rate)  ½ times number of days

  21. Cost Estimate • Forward Spectrometer: • HERMES Spectrometer magnet plus detectors • Magnet possibly available after 2007 • Large Acceptance detector: • Structure of E835 detector assumed, using HERMES figures + HERMES recoil detector • Target: • Parts of the HERMES + ANKE Targets can be recuperated ( 20% Reduction) • Infrastructure: • based on HERMES figures for platform, support structures, cablingm cooling, water lines, gas supply lines and a gas house, cold gas supply lines, electronic trailer with air conditioning

  22. Requirements for PAX at HESR PAX needs a separate experimental area • Storage cell target requires low-β section (β=0.2 m) • Polarization buildup requires a large acceptance angle at the target (Ψacc = 10 mrad) • HESR must be capable to store polarized antiprotons • Slow ramping of beam energy needed • Optimization of polarization buildup • Acceleration of polarized beam to highest energies • The experiment would benefit from higher energy (22 GeV)

  23. Final Remark Polarization data has often been the graveyard of fashionable theories. If theorists had their way, they might just ban such measurements altogether out of self-protection. J.D. Bjorken St. Croix, 1987

  24. Physics Performance • Luminosity • Spin-filtering for two beam lifetimes:P > 5% • N(pbar) =5·1011at fr~6·105 s-1 • dt = 5·1014 cm-2 • Time-averaged luminosity is about factor 3 lower • beam loss and duty cycle • Experiments with unpolarized beam • L factor 10 larger

  25. The lifetime of a stored beam is given by 20 mrad 10 5 mrad 10 mrad 8 (Target thickness = dt=5·1014 atoms/cm2) 6 beam lilfetime τb (h) 4 Ψacc = 1 mrad 2 T (MeV) 400 800 1200 Beam lifetimes in HESR In order to achieve highest polarization in the antiproton beam, acceptance angles of Ψacc = 10 mrad are needed.

  26. Low Conductance Feed Tube Method tested successfully but not optimized during development of FILTEX/HERMES Atomic Beam Source (Heidelberg 1991). H2 H1 ~3

  27. Expectedbuild-up: P(t)=tanh(t/τ1), • 1/τ1=σ1Qdtf=2.4x10-2 h-1 •  about factor 2 larger! σ1 = 122 mb (pp phase shifts) Q = 0.83 ± 0.03 dt = (5.6 ± 0.3) x 1013cm-2 f = 1.177 MHz Three distinct effects: • Selective removal through scattering beyond θacc=4.4 mradσR=83 mb • Small angle scattering of target protons into ring acceptanceσS=52 mb • Spin transfer from polarized electrons of the target atoms to the stored protons σE=-70 mb Horowitz & Meyer, PRL 72, 3981 (1994) H.O. Meyer, PRE 50, 1485 (1994) Puzzle from FILTEX Test Observed polarization build-up: dP/dt = ± (1.24 ± 0.06) x 10-2 h-1

  28. Spin transfer from electrons to protons Horowitz & Meyer, PRL 72, 3981 (1994) H.O. Meyer, PRE 50, 1485 (1994) α fine structure constant λp=(g-2)/2=1.793 anomalous magnetic moment me, mp rest masses p cm momentum a0 Bohr radius C02=2πη/[exp(2πη)-1] Coulomb wave function η=-zα/ν Coulomb parameter (neg. for anti-protons) v relative lab. velocity between p and e z beam charge number

  29. 100 T=500 MeV Goal 10 e (mbarn) T=800 MeV 1 10 100 1000 T (MeV) Antiproton Polarizer Exploit spin-transfer from polarized electrons of the target to antiprotons orbiting in HESR Expected Buildup dt=5·1014 atoms/cm2, Pelectron=0.9 antiproton Polarization (%) 10 20 30 t (h)

  30. Polarimetry Different schemes to determine targetandbeampolarization • Suitable target polarimeter (Breit-Rabi or Lamb-Shift) to measure target polarization • At lower energies (500-800 MeV) analyzing power data from PS172 are available. • Therefrom a suitable detector asymmetry can be calibrated • → effective analyzing power • Beam and target analyzing powersare identical • measure beam polarization using an unpolarized target • Export of beam polarization to other energies • target polarization is independent of beam energy

  31. Beam Polarimeter Configuration for HESR Detection system for p-pbar elastic scattering • simple, i.e. non-magnetic • Polarized Internal Storage Cell Target • magnetic guide field (Qx,Qy,Qz) • azimuthal symmetry (polarization observables) • large acceptance Ex: EDDA at COSY Storage cell

  32. Polarization Conservation in a Storage Ring H.O. Meyer et al., PRE 56, 3578 (1997) Indiana Cooler HESR design must allow for storage of polarized particles!

  33. Stored protons: P(n)=Pi()n  =(99.3±0.1)% Spin Manipulation in a Storage Ring • SPIN@COSY (A. Krisch et. al) • Frequent spin-flips reduce systematic errors • Spin-Flipping of protons and deuterons by artifical resonance • RF-Dipole • Applicable at High Energy Storage Rings (RHIC, HESR)

  34. Single Spin Asymmetries Several experiments have observed unexpectedly large single spin asymmetries in pbar-p at large values of xF ≥ 0.4 and moderate values of pT (0.7 < pT < 2.0 GeV/c) E704 Tevatron FNAL 200GeV/c π+ Large asymmetries originate from valence quarks: sign of AN related to u and d-quark polarizations π- xF

  35. Proton Electromagnetic Formfactors • Measurement of relative phases of magnetic and electric FF in the time-like region • Possible only via SSA in the annihilation pp → e+e- • Double-spin asymmetry • independent GE-Gm separation • test of Rosenbluth separation in the time-like region

  36. unknown vector coupling, but same Lorentz and spinor structure as other two processes Unknown quantities cancel in the ratios for ATT, but helicity structure remains! Cross section increases by two orders from M=4 to M=3 GeV →Drell-Yan continuum enhances sensitivity of PAX to ATT Anselmino, Barone, Drago, Nikolaev (hep-ph/0403114 v1) Extension of the “safe” region

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